Broad-area lasers (BALs) are used for optical pumping and material processing due to their wavelength selectability and high electrical to optical efficiency. However, they have both spectrally and spatially erratic emission caused by multi-mode operation, thermally induced refractive index changes, and refractive index changes induced by non-uniform gain. These effects lead to 9xx-band BALs having spectral widths from 2-6 nm at peak operating power and spectra that drift 0.3-0.4 nm per degree Kelvin. These characteristics can become problematic when strong interactions with a material's narrow absorption peak are desired (e.g., pumping ytterbium-doped fiber lasers).
Selective laser feedback has been used in the past in at least some instances and applications to modify the spectral behavior of a laser. In broad-area laser diodes, two such techniques are the inclusion of a distributed Bragg reflector on the diode or using a volume Bragg reflector in an external cavity configuration. While a distributed Bragg reflector that is monolithically integrated with the diode does not require physical alignment to narrow a BALs spectral output, such devices still suffer from wavelength variability due to temperature and, therefore, operating power. Alternatively, volume Bragg gratings are more robust to temperature changes, but must be carefully aligned and tend to have scattering losses that reduce the overall output power efficiency.